Elevatable platforms for transporting persons up and down a steep surface, along a vertical rail secured to that surface, are known. Thus for instance, German laid-open application No. 29 22 859 describes a platform sliding along a guide rail secured to a toothed rack which the platform drive engages via a pinion. This known guide rail has a complicated construction and, without its platform, cannot be used by a person.
German laid-open application No. 25 06 528 discloses a ladder for lifting a platform by means of a worm or leadscrew with turns engaging its rungs. In practice, such an arrangement would require the worm to be tilted with reference to the longitudinal axis of the ladder whose vertical stringers would then have to be far apart so that the ladder becomes very wide. Furthermore, the worm causes friction along the rungs which results in their early wear. This is especially the case with dirty ladders, like those usually found on the outside of exposed structures.